An insulated gate bipolar transistor (IGBT) device is a voltage-driven power semiconductor switching device including a bipolar junction transistor (BJT) and an insulated gate field effect transistor (MOSFET). The switching speed of an IGBT device is higher than that of a BJT transistor. Since the IGBT device is a voltage controlled device, it has the advantages of good stability, high input impedance of the MOSFET device and the low conduction voltage drop of the BJT device. Due to the advantages of the low switching losses, simple gate control, excellent switching controllability of the IGBT device, the IGBT device is widely used in the field of power electronics, e.g., in power converters in home appliances, in industrial control, in power transmission system in a vehicle, in energy power grid accesses, etc.
A conventional IGBT device includes a cell region and a terminal structure. The cell region is a function region, and the terminal structure provides the lateral voltage withstand capability of the device.
The terminal structure of the IGBT generally includes a main junction, one or more terminal rings, and a cutoff ring. The main junction is adjacent to the cell region, and the cutoff ring is located at the outermost side of the terminal structure as the terminating end. The one or more terminal rings are located between the main junction and the cutoff ring and include multiple terminal rings, each terminal ring includes a field limit ring and a multi-stage field plate.
In current mainstream terminal ring designs, the terminal rings are floating, the floating field limit rings and the field plate are used to reduce the electric field peak of the main junction to prevent an avalanche breakdown of the cell region and the main junction.
FIG. 1 is cross-sectional view of a conventional IGBT structure including a main junction and IGBT emitter having a same potential, cutoff ring and IGBT collector having a same potential. When a voltage is applied between the electrodes (i.e., between the emitter and the collector) of the IGBT device, the voltage is mainly sustained by the relatively large depth and large width of the main junction. As the applied voltage gradually increases, the depletion layer of the main junction extends outwardly along the main junction toward the terminal rings. The distance between the main junction and the first terminal ring (i.e., ring R1 in FIG. 1) is chosen so that the depletion layer of the main junction passes through the first terminal ring R1 before the avalanche breakdown of the main junction occurs. At this time, the peak electric field close to the main junction is shared by the main junction and the first terminal ring, and the peak electric field close to the main junction is reduced. The continuously increasing voltage is transmitted from the first terminal ring to the second terminal ring (i.e., ring R2 in FIG. 1) until the depletion layer of first terminal ring passes through the second terminal ring, and so on. In this way, the enhanced electric field due to the increasing voltage applied to the electrodes of the IGBT is reduced step by step through the main junction and the terminal rings, preventing the strong electric field strength from causing the avalanche breakdown of the cell region and the main junction.
However, there are inherent design flaws in the floating design of the terminal rings, that is, the peak electric field cannot be evenly distributed between the terminal rings, the maximum electric field peak congregates at the locations of the main junction and the cutoff ring.
FIG. 2 is a graph illustrating the electric field strength distribution taken along the line AA of the terminal structure of the IGBT of FIG. 1 during a normal operation. Since each terminal ring is floating, its potential is completely affected by the electric field strength transmitted by the previous terminal ring or by the main junction, and thus the terminal rings cannot most efficiently share the electric field strength. Therefore, the actual number of terminal rings required to share the electric field strength may exceed the planned number, preventing an efficient reduction of the IGBT package area and an improved reliability of the IGBT device.
Although many solutions have been proposed for adjusting the size and distance of multi-layer field plate and optimizing the structure designs of the multi-layer field plate, they cannot solve the fundamental problems of reducing the IGBT package area, cost, and improving reliability of the IGBT device.
Thus, there is a need to provide a novel IGBT terminal structure and manufacturing method thereof to overcome the above-described problems.